When a mobile communications device moves at a non-zero speed, such as in a vehicle or even when being used by a pedestrian, the downlink radio signal from the base station to the mobile communication device undergoes a process called fading. Fading appears as a relatively slow pseudo random variation of phase and amplitude of the received signal. The speed of the variation, measured as “Doppler spread”, is related to the movement speed of the mobile communication device.
Doppler spread is a different phenomenon to Doppler shift. Doppler shift is an apparent change in frequency at the mobile communication device, due to radial speed between the mobile communication device and the base station. Doppler shift is normally cancelled by the communication device's automatic frequency control loop.
A mobile communication system can be designed in such a way that an active mobile communication device estimates the Doppler spread at its antenna. The estimation is then reported to the base station, which uses this information to improve the allocation of radio resources to each individual mobile communication device. For example, the base station can opt to allocate short transmission bursts with high bandwidth and with low duty cycle to a faster moving mobile communication device, and long bursts with lower total bandwidth to slower moving mobile communication device. In both cases, the average data rate for each mobile communication device would be the same (see for example Patent Citation 1 and Patent Citation 2). However, if the mobile communication device also reports instantaneous quality of the downlink signal—such as Code Quality Indicator (CQI) in the 3GPP WCDMA or 3GPP LTE OFDM system—and the base station transmits to the high speed mobile communication device at times of its peak downlink signal (CQI) quality, the overall average reception quality can be improved.